Add validation of Kubernetes feature gates

[stoyanr]

How to categorize this PR?

/area usability /area ops-productivity /kind enhancement /platform kubevirt

What this PR does / why we need it: Adds validation of Kubernetes feature gates (in CloudControllerManager field of the ControlPlaneConfig custom resource), as requested in https://github.com/gardener/gardener/issues/3987 and https://github.com/gardener/gardener-extension-provider-gcp/issues/263. Similar to https://github.com/gardener/gardener-extension-provider-gcp/pull/280 but for kubevirt.

Which issue(s) this PR fixes: Fixes https://github.com/gardener/gardener-extension-provider-gcp/issues/263 for kubevirt.

Special notes for your reviewer:

• Also vendors g/g v1.24.1-0.20210608063816-580010846480 to be able to use the ValidateFeatureGates function.

Release note:

When creating or updating shoots, any Kubernetes feature gates mentioned are validated against the Kubernetes version. If any feature gates are unknown or not supported in the Kubernetes version, the validation fails.

[stoyanr]

/invite @rfranzke @ialidzhikov

[gardener-robot]

@rfranzke You have pull request review open invite, please check